Yes, when Young is on, he is good. But his margin for error is so thin, given he has a low 80s fastball and relies on locating in the top of the strike zone to be successful.
In regards to Young pitching in relief, I don't see him succeeding in that role, and in turn, the Mets shouldn't bring him back to be in the bullpen. He has not pitched in relief during his big league career, and his arm is more or less a high wire act anyway - he has had shoulder problems in each of the last five seasons, and he is still recovering from surgery to repair a torn capsule in his shoulder from last May. In a five or six man rotation, Young's outings are predictable, and he can prepare according to a set program. In a bullpen role, the preparation changes as well as the predictability and frequency of his outings, and I just don't see Young being able to get ready under those conditions.
If the Mets bring Young back next season, it will be for starting pitching depth behind Johan Santana, R.A. Dickey, Matt Harvey, Jon Niese, and Dillon Gee. It's worth noting the Mets also have Jeremy Hefner and maybe Collin McHugh as rotation depth next season. In addition, the Mets could non-tender Mike Pelfrey in December, bring him back on a minor league deal, let him finish his rehabilitation from Tommy John Surgery with the Mets, and add him to the mix in May or June. The latter isn't a guarantee, but something that might happen before the Mets consider Young for rotation depth, especially with Zack Wheeler likely to be making noise next Spring and in the early part of the season.